The present invention relates to an image reader capable of reading an image with an image sensor for converting a read image into an electric video signal and transferring a video signal obtained by digitizing the electric video signal with an A/D converter to an external unit and its control method and an image read method.
An image reader (hereafter referred to as image scanner) has been known so far, which reads a manuscript with an image sensor to convert it into an electric video signal and moreover, converts the analog signal into a digital signal with an A/D converter to transfer it to an external unit.
This type of the image scanner has a home-position sensor (hereafter referred to as HP sensor) for detecting the position of an image sensor in accordance with on/off of the output of a photodiode or the like when moving the image sensor for scanning a manuscript to a read start position (hereafter referred to as home position) in order to determine an image read start position and determines the read start position with the home position sensor.
Moreover, a product has been recently marketed which detects a home position (hereafter referred to as HP detection) without using the above photodiode in order to cut down the cost of a scanner. In the case of this type of the product, a specific pattern is set to the read start position of a scanner to decide a reference position by detecting the pattern.
For example, a white plate and a black plate are arranged out of an image region adjacently to each other and the region is read by an image sensor to detect the boundary between the white plate and the black plate in accordance with the output of the image sensor and perform HP detection.
Moreover, it is also performed to combine detection of the above pattern with a HP sensor, move an image sensor up to the position of the HP sensor, and then detect the boundary between a white plate and a black plate.
However, the above detection methods have the following problems.
(1) The gap between a pressure plate and a housing (a body of a scanner) may be erroneously detected as the boundary between a white plate and a black plate.
(2) Because external light comes in from the gap between a pressure plate and a housing, the boundary between a white plate and a black plate is not accurately detected.
(3) When using a HP sensor, HP detection cannot be made if the HP sensor is damaged or external light enters the read region of an image scanner.
(4) By using a complex pattern as a specific pattern, accurate HP detection can be made. However, when using an inexpensive scanner, it is necessary to perform HP detection only from a simple black region in order to further cut down the cost before setting a complex pattern. However, the processing section (CPU) for performing the detection frequently has a low performance and therefore, it is difficult to decide whether pattern data values are matched each other at a high speed.
Moreover, it is limited to perform complex processing. Therefore, erroneous detection may occur depending on the manuscript on a manuscript table and an image sensor may stop at a position other than a reference position.